1. Field of the Invention
The present invention relates to a wireless communication system based on frequency division multiple access. More particularly, the present invention relates to a method and apparatus for multiplexing and transmitting data and control information in a wireless communication system based on frequency division multiple access.
2. Description of the Related Art
Recent developments in broadcasting and mobile communication systems technology has led to the wide use of an Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme. The OFDM scheme eliminates the interference between multi-path signals, which is frequently found in wireless communication channels. Also, the OFDM scheme guarantees the orthogonality between multiple access users and facilitates an efficient use of resources. Therefore, the OFDM scheme is available for high speed data transmission and broadband systems more than the conventional Code Division Multiple Access (CDMA) scheme. However, the OFDM scheme is a multi-carrier transmission scheme, in which transmission data is distributed to multiple sub-carriers and is then transmitted in parallel. This causes the OFDM scheme to increase the Peak-to-Average Power Ratio (PAPR) of the transmission signals.
A large PAPR causes distortion of output signals in a Radio Frequency (RF) power amplifier of a transmitter. Therefore, in order to solve such a problem, the transmitter requires power back-off to reduce the input power to the amplifier. Therefore, when the OFDM scheme is applied to the uplink of a mobile communication system, a terminal must perform the power back-off for the transmission signals, which results in the reduction of the cell coverage.
Interleaved Frequency Division Multiple Access (IFDMA) is being actively researched as a solution to solve the PAPR problem of the OFDM technology. The IFDMA guarantees the orthogonality between the multiple access users like the OFDM and is a technology based on a single sub-carrier, which shows a very low PAPR of transmission signals. Applying the IFDMA to a mobile communication system reduces the problem of cell coverage reduction due to the PAPR increase.
FIG. 1 illustrates a structure of a typical IFDMA transmitter.
Although the structure shown in FIG. 1 uses a Fast Fourier Transform (FFT) unit 104 and an Inverse Fast Fourier Transform (IFFT) unit 106, exemplary embodiments of the present invention are not limited to the shown structure and can be implemented by additional structures. The implementation that uses the FFT unit 104 and the IFFT unit 106 is advantageous because it facilitates an easy change of IFDMA system parameters without a high hardware complexity.
The OFDM and the IFDMA may have the following differences in the aspect of transmitter structure. In addition to the IFFT unit 106 which is used for multi-carrier transmission in the OFDM transmitter, the IFDMA transmitter includes the FFT unit 104 located before the IFFT unit 106. Therefore, the transmission modulation (TX) symbols 100 in FIG. 1 are input to the FFT unit 104 block by block, each of which includes M number of transmission modulation symbols. The block is referred to as “symbol block,” and the period at which the symbol block is input to the FFT unit is referred to as “symbol block period.” The signals output from the FFT unit 104 are input to the IFFT unit 106 at equal intervals, so that the IFDMA transmission signal elements are transmitted in the frequency domain by sub-carriers of equal intervals. In this process, it is usual for the input/output size N of the IFFT unit 106 to have a larger value than that of the input/output size M of the FFT unit 104. In the OFDM transmitter, the transmission symbol blocks 100 are directly input to the IFFT unit 106 without passing through the FFT unit 104 and are then transmitted by multiple sub-carriers, thereby generating a PAPR with a large value.
In the IFDMA transmitter, the transmission symbols are pre-processed by the FFT unit 104 before being processed by the IFFT unit 106. This occurs even though the transmission symbols are finally processed by the IFFT unit 106 before being transmitted by multiple carriers. The pre-processing of the transmission symbols makes it possible, due to the counterbalancing between the FFT unit 104 and the IFFT unit 106, to have an effect similar to that which occurs when the output signals of the IFFT unit 106 are transmitted by a single sub-carrier, thereby achieving a low PAPR. Finally, the outputs of the IFFT unit 106 are converted to a serial stream by a Parallel-to-Serial Converter (PSC) 102. Before the serial stream is then transmitted, a Cyclic Prefix (CP) or guard interval is attached to the serial stream as it is in the OFDM system, to prevent interference between multi-path channel signal elements.
FIG. 2 illustrates a structure of a transmitter based on a Localized Frequency Division Multiple Access (LFDMA) technique, which is similar to the IFDMA technique. The LFDMA technique also guarantees the orthogonality between multiple access users, is based on single carrier transmission, and can achieve a PAPR lower than that of the OFDM. As illustrated in FIGS. 1 and 2, the difference between the LFDMA and the IFDMA in the view of transmitter structure is that the outputs of the FFT unit 204 turn into inputs to the IFFT unit 206, which have sequential indexes following the last index of the FFT unit 204. In the frequency domain, the LFDMA signals occupy the band constituted by adjacent sub-carriers used when the outputs of the FFT unit 204 are mapped into the inputs of the IFFT unit 206. In other words, in the frequency domain, the IFDMA signals occupy the sub-carrier bands (sub-bands) distributed at an equal interval, and the LFDMA signals occupy the sub-band constituted by adjacent sub-carriers.
In order to apply the IFDMA and LFDMA based systems to a broadcasting or mobile communication system, it is necessary to transmit data and to control information and a pilot signal for demodulation and decoding of the data in a receiver. The pilot signal has a guaranteed pattern between a transmitter and a receiver. Therefore, when a received signal has a distortion due to a wireless fading channel, the receiver can estimate and eliminate, based on the pilot signal, the distortion in the received signal due to the wireless fading channel. The control information includes a modulation scheme applied to the transmitted data, a channel coding scheme, a data block size, and Hybrid Automatic Repeat Request (HARQ)-related information such as a serve packet ID. By receiving the control information, the receiver can understand the information applied to the transmitted data to perform various operations including demodulation and decoding of the received data.
According to the CDMA technique widely applied to current mobile communication systems, the data, control information, and pilot signal are transmitted by using different channelization codes. This allows the receiver to separate and detect the signals without interference. According to the OFDM technique, the data, control information, and pilot signal are transmitted by different sub-carriers or after being temporally divided.
Since the control information is not a large quantity of information capable of totally occupying one time slot, application of the time division-based multiplexing scheme may result in an unnecessary waste of resources. When the control information is transmitted by a separate sub-carrier different from that which carries the data as it does in the OFDM scheme, it is problematic in that the transmitted signal has an increased PAPR.
Accordingly, there is a need for an improved method and apparatus for multiplexing data and control information to lower a PAPR of a transmitted signal and to facilitate resource efficiency in an IFDMA or LFDMA-based communication system.